EP1001256A1, which is incorporated by reference herein, discloses a torque sensor 100 in which two disks 101, 102 are mounted in close proximity and attached to the respective ends of a torque transmitting shaft. Both of the discs 101, 102 carry a set of circumferentially spaced slots 103, 104, and the slots in each of the two discs co-operate to define apertures for the passage of light. A light source 10 is provided to one side of the first disk to emit light through the slots in the first disk and the slots in the second disk onto an optical detector 106 provided on the other side of the disks. As torque is applied to the torque transmitting shaft relative motion between the two discs alters the way in which the slots overlap and hence the size of the apertures which control the pattern of light incident upon the optical detector.
The optical detector comprises a one-dimensional array of detector elements and the output from the array is passed to a processor operative to determine the relative positions of the two discs by determining the relative positions of transitions from light to dark in the pattern formed on the detector array. Each transition corresponds to an edge of a slot in one of the discs. In practice, five edges must be imaged onto the linear array in order to unambiguously determine the relative position of the two discs. This provides an indication of the torque applied to the shaft corresponding to the received pattern on the array.
If the torsion bar is omitted, the two disks allow the device to be used as simple angular displacement sensor. In a further modification, a single disk may be provided which results in a simple rotary position sensor. The present invention relates to all three types of sensor
In a linear array sensor the torque is typically derived from the relative angular position of wide and narrow spokes on two modulating disks. Unfortunately, a problem can arise with a torque sensor (or angular displacement or rotary position sensor) of this type if the two discs are not accurately aligned. This may occur due to bending of the torque transmitting shaft or perhaps due to misalignment during manufacture of a sensor. If only a single linear array is used, the effect is for the slots of one (or both) disks to move longitudinally along the array. The run out due to this type of misalignment will produce a sinusoidal variation in the measured torque over a complete revolution of the discs which cannot be differentiated with a single array.
EP 1001256 A1 teaches a solution to this problem. Two light sources 105, 107 are provided which are arranged at diametrically opposed locations on the disks with each light source transmitting light through the apertures defined by the two disks onto a respective linear array 106, 108. A prior art sensor of this type is illustrated in FIG. 1 of the accompanying drawings.
Each linear array 106, 108 provides an output indicative of the light pattern formed on the array to a processor which calculates torque value from each pattern. The sinusoidal error due to run-out can then be compensated by taking the average of the two torque values. This effectively makes the sensor immune to disk run-out errors.
For example, FIG. 7c of the accompanying drawings shows the position of two arrays 300, 301 on opposing sides of the modulating disks. Two wide spokes 302, 303 are imaged onto the first array 300 and two more wide spokes 304, 305 onto the second array 301. On each array a narrow spoke 306, 307 is also imaged.
Similarly, FIG. 7(b) shows the same spokes imaged onto the two arrays. In FIG. 7(a) the narrow spoke disc is rotated relative to the wide spoke disk due to an applied torque but in FIG. 7(b) the arrays xe2x80x9cseexe2x80x9d the same result due to run out of the narrow spoke disc with no applied torque.
Since two arrays are provided, the results of the determined torque can be arranged to remove the effect of the run-out, and distinguish FIG. 7(a) from FIG. 7(b). Nevertheless, an error may still exist since it is necessary to determine the torque by performing an arctan calculation to convert the linear array measurements into angular measurements. This requires the exact centre of the modulating disks to be assumed to pass through the centre of the linear arrays. Clearly, this is not in fact true where run-out is present and so performance is degraded.
The provision of linear arrays at two diametrically opposed positions in the sensor increases the overall cost of the design. The relative location of each of the arrays must be maintained with great accuracy throughout the life of the device. Separate connections from the two arrays to the processor are required and the number of light sources is also doubled compared to a simple one-array device.
It is the object of this invention to ameliorate some of the problems associated with the prior art displacement and torque sensors.
In accordance with a first aspect the invention provides an optical displacement sensor comprising a source of optical radiation, an array of radiation detectors, at least one modulating element having alternating first and second modulating regions circumferentially spaced around a central axis of the element, the first and second regions having different optical characteristics and the transition between adjacent first and second regions being defined by a substantially radially extending edge, the modulating element being displaceable relative to the array of detectors so that the first and second regions are exposed to optical radiation from the source and pass by the detector array to form an image of said first and second regions of the modulating element on the array,
a data processor connected to the detector array to receive therefrom respective signals dedpendent upon the of radiation falling on the detectors,
and characterised in that:
the detector array comprises a two-dimensional array of detector elements which produces a two-dimensional image of the first and second regions;
the processor is adapted to identify the orientation of at least two different radially extending edges of regions on the at least one modulating element from the two-dimensional image and to determine the position of the centre of the element from the determined orientation of the edges.
The signals produced by the data processor are most preferably dependent upon the intensity of radiation falling upon the detectors.
In the prior art a single one-dimensional array is provided which allows the presence of an edge to be identified but not its orientation in space. This does not allow the position of the centre of the first element to be determined.
The modulating element may include a third modulating region that is distinct from the first and second modulating regions (i.e. wider, different optical characteristics). This can be used as a position index spoke.
Preferably the processor is adapted to identify in the two dimensional image at least two radially spaced portions of each identified edge from the image captured by the detector array.
The processor may be adapted to determine the orientation of a detected edge by generating a vector which passes through the two identified portions.
By two dimensional array we mean an array which can image at least two different radially spaced portions on an edge of a modulating region.
By providing a two dimensional array it is possible to capture at least two radially spaced points of the same edge and hence determine the orientation of the edge.
The two dimensional array may conveniently comprise two sub-arrays with each sub-array comprising a linear array of detector elements. The two arrays may be substantially identical. They may be arranged in parallel and in close proximity on one side of the modulating element central axis. Most conveniently, to assist in detection of edges which are imaged onto both sub-arrays the spacing between the detectors of the two arrays is smaller than the angular spacing between the edges that are to be identified. Other arrangements include, for example, a 128xc3x9716 array in which it is possible to detect 16 different radially spaced portions on an edge of a modulating region.
Where two sub-arrays are provided, each array may be adapted to generate a respective sub-image which is passed to the processor. Each of the two sub-images corresponds to a different portion of the first and second region. Preferably, the sub-images are captured at the same instant in time, or at substantially the same instant in time.
The processor may comprise means for identifying the orientation of the edges by identifying the position of an inner portion of a first edge of the modulating regions in the first image and the position of an outer portion of the first edge in the second image,
means for identifying the position of an inner portion of a second edge of the modulating regions in the first image and the position of an outer portion of the second edge from the second image,
orientation determining means for determining the orientation of the two edges from the relative positions of the portions in the first and second images; and
position determining means for determining the position of the centre of the modulating element from the determined orientation of the two identified edges.
A pair of closely spaced linear arrays thus provides sufficient information to permit the position of the modulating element to be identified by determining the orientation of two different edges of the modulating regions on the desk. This enables errors in the centre output due to run out of the modulating element to be compensated. It also permits the optical radii of the modulating element to be derived.
The processor may be adapted to determine the centre of rotation of both of the first and the second elements by identifying the orientation of at least two edges on each of the elements.
Each first region of the first and second modulating elements may comprise a radially extending slot formed between circumferentially spaced radially extending edges.
The second regions between the slots may be opaque. This arrangement is preferred as it provides the maximum intensity difference between the light and dark portions of the image on the array. It is the transition from one intensity level to the other in the image that is used to identify the position of an edge.
It is preferred that the spatial extent of the two-dimensional array is such that, in use, at least five transitions between first and second intensity thresholds will always be detectable by each array. This corresponds to the detection of at least five edges.
The light source and the detector array may be provided on opposite sides of the modulating element to form a transmissive type sensor. Alternatively, the light source and the detector array may be provided on the same side of the modulating elements. In the first case light from the source is either passed through to the detector array or blocked by the modulating regions. In the second case, light from the source may be either reflected from the modulating regions onto the detector array or passes through the modulating regions away from the detector array.
The skilled man will understand that many modifications to the sensor are possible within theses two broad types.
It is preferred that the light source is at least partially diffuse. Alternatively, a point source may be used in combination with a diffuser provided in front of the light source.
First and second modulating elements may be provided. Each may have first and second modulating regions which overlap, the first modulating element being displacable relative to the second modulating element. They may be connected at axially spaced locations along a torsion bar.
The first modulating element may be attached to an input shaft and the second modulating element may be attached to an output shaft, the input and output shaft being connected by a torsion bar. In this arrangement a torque applied to the torsion bar will produce the relative angular displacement of the first and second elements.
A sensor of this type allows torque to be measured.
Thus, in accordance with a second aspect the invention provides a torque sensor comprising a displacement sensor according to the first aspect of the invention in which the first element and the second element are connected by a torsion bar.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.